Your car has many ways to let you know it needs Coolant. One way is a warning light on the dash. Other ways are your heater stops blowing hot air. The most severe is the engine temp starts to go over normal. Other ways you can tell if your car needs Coolant. Open the hood and look at the Coolant Reservoir. If you have an Empty Coolant Reservoir, then your car needs coolant. Sometimes you need to Add Coolant To The Reservoir. A Full Coolant Reservoir will have the coolant or Antifreeze level at the full mark. The best time to add coolant is when the Engine is cold. This could take from 1 to 4 hours. Depending on the engine and outside temperature.